Mice bearing a large s.c. MOPC-315 tumor can be cured by a dose of cyclophosphamide (CY) ranging from 15 to 200 mg/kg. However, the low (15 mg/kg) and the high (200 mg/kg) doses mediate tumor eradication via different mechanisms. Tumor eradication by the low dose of drug requires the cooperation of the toxic effect of the drug and T-cell-dependent antitumor immunity. On the other hand, tumor eradication by the high dose of drug does not require the participation of antitumor immunity but depends primarily on the tumoricidal activity of the drug. Spleen cells from tumor-bearing mice treated with the low dose of CY exhibit an augmented antitumor immune potential, whereas spleen cells from tumor-bearing mice treated with the high dose of CY exhibit suppressed antitumor immune potential. More importantly, tumor-bearing mice treated with the low dose of drug are able to reject a challenge with 300 times the minimal lethal tumor dose given 1, 6, or 31 days after CY therapy, whereas mice treated with the high dose of drug are unable to reject such a challenge given within the same time intervals after CY therapy. Moreover, when mice bearing a large tumor are treated with the high dose of CY and subsequently challenged again with tumor cells to establish a Day 4 nonpalpable tumor, this tumor is less responsive to cure by combined chemoimmunotherapy than is a Day 4 nonpalpable tumor established in normal mice. Thus, although the high dose of CY can cure most mice bearing a large-size MOPC-315 tumor, it not only does not result in antitumor immunity, but it actually reduces the effectiveness of chemoimmunotherapy for a second tumor challenge. In contrast, mice cured with the low dose of CY exhibit long-lasting potent antitumor immunity.